1. Field of the Invention
The present invention relates to algae-resistant roofing granules. More particularly, it relates to colored roofing granules containing metallic copper powder that resists discoloration by terrestrial algae.
2. Reported Development
Roofing granules, both natural and artificially color-coated granules, are extensively used in roll roofing and asphalt shingle compositions. The roofing granules are typically embedded in the asphalt coating on the surface of an asphalt-impregnated felt base material, the granules thus forming a coating that provides a weather-resistant exterior roofing surface. In addition to its utility, the granule coating also provides an aesthetic effect observable with respect to the coating composition, the appearance of the granules is of major marketing interest. For this reason a pigmented color coating is ordinarily applied to the base mineral granules to enhance their visual, decorative effect. The method of providing artificially colored roofing granules involves crushing and screening minerals and applying to the so-obtained granules a coating containing the desired pigment. The granules and the pigment are bonded together by using a soluble silicate binder solution which is then insolubilized by heat treatment alone or by chemical action to a water-insoluble state. The function of the soluble silicate is to first uniformly distribute and encapsulate the pigment within the coating matrix and then to bind the pigment and granules together when the silicate is insolubilized.
The soluble silicate binder is converted to an insoluble state by heat treatment as in the case of a silicate/clay coating matrix or by a combination of heat treatment and chemical action such as by the addition of an acidic material to the fired soluble silicate.
Examplary references directed to the production of artificially colored granules and stabilizing the pigment therein are as follows.
U.S. Pat. No. Re. 20,295 describes artificially colored roofing granules produced by: depositing on granular mineral matter a metal salt followed by the addition of a soluble silicate which will react to form an insoluble metal glass; and subjecting the insoluble metal glass to heat in order to fuse the metal silicate on the granules.
U.S. Pat. No. 2,001,448 describes a method of producing artificially colored granules by the steps of: providing base particles, such as sand, gravel, rock, blast-furnace slags, or burned clay or shale and crushed brick; mixing the particles with a hydrous plastic clay in either wet or dry condition so that the clay coats the exterior surfaces of the particles; adding a suitable color pigment to coat the particles therewith; drying the particles; adding silicate of soda either per se or with a coloring pigment therein to react with the clay coating covering the particles; and heating the particles to a temperature of about 450° to 1200° F. to fuse together the coatings and the particles.
U.S. Pat. No. 2,695,851, describes artificially colored roofing granules and a method for producing them. The method comprises: forming on the base granules a coating of film-forming composition which contains a pigment, an alkali silicate solution, finely-divided hydrated alumina, and an insolubilizing agent comprising of clay, cryolite, feldspar, aluminum fluoride and sodium fluorosilicate; and heating the so-obtained granules to about 1000° F.
The hydrated alumina used in combination with a light-colored, opaque pigment, such as titanium dioxide produces lighter and brighter granules than can be produced with titanium dioxide in the absence of hydrated alumina.
U.S. Pat. No. 2,732,311 is directed to a coating process for producing radiation-reflective roofing granules.
In the process the raw granules are mixed with a clay-like material, such as kaolin at a temperature below 100° F., preferably at room temperature. A solution of sodium silicate is added to the mixture. Alternatively, the clay and sodium silicate can be pre-mixed then applied to the granules. The granules are then partially air-dried to develop a tacky film on the granules. When the proper viscous tacky film has been formed, aluminum flakes are then added to the mixture and are caused to adhere to the coated granules by further mixing. Instead of aluminum flakes, copper flakes, brass flakes, or metallic particles known as bronze powders and aluminum bronze may be used. The metal flake-coated granules are then fired at a temperature of about 900° to 1000° F.
U.S. Pat. No. 2,981,636 describes artificially colored roofing granules and a method for their preparation. The method of producing the colored roofing granules involves the steps of:                mixing the cleaned and heated granules with an aqueous solution of sodium silicate; drying and dehydrating the mixture of granules and the aqueous solution; and adding an insolubilizing agent containing a desired pigment therein to the dehydrated granules.        
The insolubilizing agent is a mixture of aluminum chloride and ammonium chloride, however, other insolubilizing agents may also be used, such as aluminum sulfate, magnesium chloride, zinc chloride, and hydrochloric acid.
U.S. Pat. No. 3,494,727 discloses a microbe resistant roof wherein the roofing material contains embedded therein visually observable metallic chips, such as copper, lead, zinc and iron.
U.S. Pat. No. 3,507,676 discloses zinc-containing coating for roofing granules in the form of zinc, ZnO or ZnS.
U.S. Pat. No. 3,528,842 discloses copper compound-containing algicidal surfacing, the copper compound being cuprous oxide or cuprous bromide.
U.S. Pat. No. 3,598,627 discloses fungus-resistant shingles the outside surface of which are asphalt coated and are covered by a hard, granular weathering material and granular zinc.
U.S. Pat. No. 3,884,706 discloses algicidal roofing granules coated with a substantially water insoluble silicate-clay coating into which small amounts of algicides, e.g. 0.05% to 0.5% zinc algicide and 0.05% to 0.5% copper algicide, are incorporated based on the total weight of the roofing granules.
U.S. Pat. No. 4,092,441 discloses roofing granule treatment by coating the granules with a metallic algicide, such as zinc, copper, nickel and mixtures thereof. The metals are sprayed onto the granules in the form of molten droplets.
U.S. Pat. No. 5,382,475 discloses algicidal granules having a ceramic coating thereon comprising three layers, the first two of which have a copper compound, and the third layer has a pigment, a borate compound and zinc oxide.
U.S. Pat. No. 3,894,877 discloses incorporating copper silicate into color coated roofing granules using a heavy processing oil. The copper silicate adheres to the surface of the granule color coating with the finer particles thereof being adsorbed into the color coat with the processing oil.
U.S. Pat. No. 6,214,466 discloses algae-resistant roofing granules coated with: a first coat consisting of a fired silicate-clay matrix containing cuprous oxide and zinc sulfide to provide a slow, long-term bimetallic copper and zinc ions release; and a second coat consisting of a fired silicate-clay matrix containing a pigment.
An object of the present invention is to provide algae-retardant, artificially colored roofing granules.
Another object of the present invention is to enhance algicidal activity of the algicide-containing artificially colored roofing granules.
Still another object of the present invention is to reduce the cost of producing algae-resistant, long-term and slow-release algicide-containing granules.